The present invention relates to a lyotropic liquid crystal composition comprising silver grains, silver halide grains or optically anisotropic grains dispersed in lyotropic liquid crystal. The invention also relates to an optically anisotropic thin film comprising the grains and the lyotropic liquid crystal. The invention further relates to a process for preparation of the optically anisotropic thin film by use of the lyotropic liquid crystal composition.
A diffraction optical device, which is produced on the basis of lithography and etching technology, has been rapidly improved according as the technology has been developed. For example, a diffraction optical device having a pitch shorter than visible wavelength has been studied. Generally, a material having a structure smaller than visible wavelength can be regarded as a homogeneous media having a certain refractive index, which depends on the structure and the refractive index of the material alone. The diffraction device having a fine pitch has an important advantage of controlling polarization. In fact, if the fine structure of the device is not the same in all directions, the device shows optical anisotropy called xe2x80x9cstructural birefringencexe2x80x9d. Because of this character, it is theoretically possible to produce a diffraction grating showing controlled polarization. The fine diffraction device has been studied since late 1980s. The studies are described in Kikuta et al, OPTRONICS, 8(1996), 132.
A practically used polarizing element in the above technical field comprises silver grains of football shape dispersed in glass (described in Japanese Patent Publication No. 2(1990)-40619, U.S. Pat. Nos. 4,486,213 and 4,479,819). This device is prepared in the following manner. First, a glass material containing silver and halogen is subjected to heat treatment so as to deposit silver halide grains. The material is then heated and stretched to make the grains football shape, and thereby optical anisotropy is caused in the silver halide grains. Finally, the material is heated under reducing atmosphere to reduce the silver halide into silver metal.
In the thus-prepared device, the silver halide grains do not have uniform aspect ratio (ratio between the lengths of long and short axes). Further, it is difficult to fully reduce the silver halide in the glass, and consequently opaque silver halide slightly remains.
In order to solve these problems, it is proposed to produce a polarizing element through a film-forming process such as vacuum deposition or spattering process (described in Resume for Japan Electronic Information and Communication Society, autumn meeting 1990, C-212). According to the proposed process, first a metal layer is formed by vacuum deposition on a dielectric substrate such as a glass plate. On the formed layer, a dielectric layer such as a glass layer is then formed by, for example, spattering. The procedure is repeated several times to form some metal and dielectric layers piled up alternatively. The formed layered composition is heated and stretched so that the metal layers may be transformed into discontinuous islanded metal particle layers. Since each metal particle in the layers is stretched to transform into football shape, the polarization is realized.
For improving efficiency of light used in a polarizing plate, it is proposed to use a polarizing plate of light-scattering type in place of or in addition to that of light-absorbing type. The polarizing plate of light-scattering type as well as that of light-absorbing type transmits only the light component polarized parallel to the polarizing axis. However, the plate of light-scattering type does not absorb but scatters forward or backward the perpendicularly polarized component, and accordingly it improves the efficiency of light.
The polarizing plate of light-scattering type is described in Japanese Patent Provisional Publication Nos. 8(1996)-76114, 9(1997)-274108, 9(1997)-297204, Japanese Patent Publication Nos. 11(1999)-502036, 11(1999)-509014, U.S. Pat. Nos. 5,783,120, 5,825,543 and 5,867,316.
An anisotropic thin film comprising fine nickel metal rods is reported (Saito et al, Appl. Phys. Lett., 55(1989), No. 7, 607). In preparing the film, a porous alumina thin film is electrochemically formed on a cathode, and then the porosities are filled with nickel metal. The thus-formed film shows such polarizing performance that the extinction ratio is 30 dB at the wavelength of 1.3 xcexcm.
The optical characteristics of gold colloid have been studied for a long time. For example, a monodispersive colloid of uniform fine gold rods is reported (van der Zande et al, J. Phys. Chem. B, 101(1997), 852). In preparing the colloid, a porous alumina film is formed by anode oxidation (diameter of porosity: 12 nm). In the film, gold rods are grown by electrochemical deposition from a gold solution. The alumina film is then removed to obtain the dispersive fine gold rods. The lengths of the rods are controlled in the range of 12 to 160 nm by the time for deposition. The anisotropy of the gold rods depends on the ratio of length/diameter, and accordingly the spectrum remarkably varies according to the ratio.
Gabor L. Hornyak et al. also adopt the method in which fine porosities are charged with gold to prepare various alumina films containing fine gold rods, and study the optical characters of the films containing anisotropically aligned fine gold rods having various aspect ratios (J. Phys. Chem. B, 101(1997), 1548). As a result, they confirm that Maxwell-Garnet theorem, which is a relation between colloidal particles and plasmon resonance absorption, holds for these fine gold particles.
Kikuta et al. notice an intense dispersion on effective refractive index of structural birefringence based on the above-described fine aligning structure. They suggest that this phenomenon can be utilized to produce a wide-ranging xcex/4 plate (Resume for Japan Appiled Physics Society, autumn meeting 1990, 26a-SP-22, 807).
Giving nonlinear optical effects, the composite material containing dispersed structural units of nanometer size (e.g., metal particles, semiconductor crystallites) has been studied to use in the field of nonlinear optics.
The term xe2x80x9cnonlinear optical effectsxe2x80x9d means the following phenomena. When a ray having the electric field E and the frequency xcfx89 comes into the material, the electric field (E) induces alternative separation between positive and negative electric charge at the frequency xcfx89. This alternative charge separation is called xe2x80x9cpolarization wavexe2x80x9d. The polarization wave then functions as a wave source to cause a ray of the frequency xcfx89, which comes out of the material. Consequently, the incident ray and the ray coming out have the same frequency. This is a normal interaction between light and matter. However, in some materials, when the incident ray having the electric field (E) and the frequency xcfx89 comes, another polarization wave is induced in proportion to the power of E. These materials are called xe2x80x9cnonlinear optical materialsxe2x80x9d. The nonlinear optical material gives peculiar phenomena. For example, the ray coming out of the material has a frequency of twice or more as large as the incident frequency xcfx89 (namely, the color of the ray coming out is different from that of the incident ray). Further, the refractive index of the material varies according to the square of the intensity of light (electric field). These peculiar phenomena are generally called xe2x80x9cnonlinear optical effectsxe2x80x9d. The nonlinear optical effects have been studied in view of application to wavelength conversion of lasers or optical logic devices. There is a close relation between the nonlinear optical effects and the quantum confinement. In fact, if a material comprises fine metal or semiconductor particles of nanometer size, the quanta (such as electrons, positive holes and excitons) concerned with the interaction between light and matter cannot freely behave and consequently induce the peculiar phenomena that are not observed in a normal bulk state. In this way, the quantum confinement is known to cause the intense nonlinear optical effects, and therefore media containing dispersed fine particles or materials having fine structural units of nanometer size have been noticed and studied to use as nonlinear optical materials.
For example, a nonlinear optical composite material containing dispersed particles of nanometer size is disclosed in NEW GLASS, 3(1989), No. 4, pp. 41. For producing the material, glass and particle material of nanometer size are melted and mixed, and then the mixture is subjected to heat treatment at a proper temperature to deposit the particles in the glass. Another nonlinear optical composite material containing dispersed particles of nanometer size is disclosed in Hikari Gijutsu Contact (written in Japanese), 27(1989), No. 7, pp. 389. For producing the material, glass and particles of nanometer size are simultaneously deposited on a substrate so that the particles may be dispersed in a thin film of glass, and the film is then subjected to heat treatment.
There are some publications reporting that lyotropic liquid crystal molecules themselves are aligned when they are sheared. Gudrun Schmidt et al. report that amphiphilic molecules of C12H25(OC2H4)6OH, which serves as a nonionic surface-active agent, are aligned along the flow direction (Journal of Physical Chemistry B, 102(1998), 507). Quist et al. report that amphiphilic molecules of sodium dodecybenzenesulfonate, which serves as an anionic surface-active agent, are aligned to form a lamellar structure (Liquid Crystals, 16(1994), 235).
According to Stefan Muller et al. (Langmuir, 15(1999), 7558), if the shearing speed is relatively low the molecules of C12H25(OC2H4)4OH are aligned to form a lamellar phase so that the normal of the phase may be parallel to the velocity gradient. On the other hand, in middle or more velocity range of the shearing speed, the molecules form a multi lamellar vesicle.
There are some attempts to align aqua-soluble dye molecules in a certain direction. These attempts are made with the aim of using the dye for a polarizing membrane. For example, Japanese Patent Provisional Publication No. 10(1998)-333154 (Ichimura et al.) describes an experiment about aligned dye molecules. In the experiment, first poly(4-methacryloylazobenzene) is spin-coated and exposed to light so as to align the molecules to form an orientation layer. An aqueous solution of Direct Blue 67 is then cast on the orientation layer. According to the publication, the dye molecules are aligned vertically to the optical axis of the applied rays. Croeley et al. (Colloid and Surfaces A, 129-130(1997), 95) report that azo dye molecules, which form a hexagonal phase in aqueous solution, are aligned highly in order along the flow direction at a low shearing speed (2.78/s). Further, they also report that cyanine dye molecules are aligned to form a lamellar phase so that the normal of the phase may be parallel to the velocity gradient.
It is also reported that a lyotropic liquid crystal aqueous solution comprising discotic dye molecules of surface-active agent type is coated so that the molecules may be shared and aligned so as to form a polarizing membrane. Bobrov et al. (Mat. Res. Soc. Symp. Proc., 508(1998), 225) coat and share an aqueous solution of anthraquinone dye containing some additives (e.g., Trition-X-100, hydroquinone, polyethylene glycol) to produce a blue liquid crystal membrane giving polarization of 95%. They also produce a gray polarizing membrane from the solution containing some kinds of discotic dye molecules.
In the above-described studies, lyotropic liquid crystal molecules are shared and aligned in a host/guest structure. The structure is constituted of the lyotropic liquid crystal molecules alone or a combination of the liquid crystal molecules as the host and other organic molecules as the guest. However, it is unknown that, in order to induce anisotropy, the lyotropic liquid crystal molecules are shared and aligned in a host/guest structure in which anisotropic inorganic, organic or inorganic/organic composite material of micro- or nano-meter size are contained as the guest.
In a media containing dispersed fine structural units of nanometer size (e.g., fine particles), each unit must enhance each other""s nonlinear optical effect to give both intense nonlinear optical effects and effective optical anisotropy.
However, in the above-described known composite materials containing dispersed particles of nanometer size, the orderliness of the aligned particles is too low. Further, it is almost impossible to produce a device of the material having a large surface through a simple process.
Furthermore, there are some practical problems in the producing process. Since most inorganic or composite particles generally have large surface energy, they are liable to aggregate when they are taken from a mold in the production process. It is, therefore, very difficult to clearly separate the rod-like or tabular particles from the mold. Generally, it is not easy to handle the fine particles of micro- or nano-meter size in the same manner as normal fine powder. Further, it is difficult to evenly disperse and coat the particles and hence to prepare a stable dispersion.
The applicants have studied a method by which an isotropic or anisotropic media in which anisotropic materials of nanometer size are stably formed in the presence of the lyotropic liquid crystal can be easily separated and removed from the liquid crystal and the materials. As a result, the applicants have finally achieved the present invention. In the invention, rod-like or tabular grains (silver grains, silver halide grains or optically anisotropic grains) are stably formed and dispersed in protective colloid of gelatin. The silver halide grains or the silver grains and lyotropic liquid crystal are mixed, and the protective colloid (gelatin) is decomposed with enzyme to prepare a composition. In the prepared composition, the rod-like or tabular silver halide grains or reduced silver grains thereof are stably dispersed in the lyotropic liquid crystal. The composition is coated so that the grains may be shared and anisotropically aligned, and thereby an optically anisotropic material is easily produced.
An object of the present invention is to form an optically anisotropic thin film from a stable lyotropic liquid crystal composition.
The present invention provides a lyotropic liquid crystal composition comprising silver halide grains or silver grains dispersed in lyotropic liquid crystal, said grains having an aspect ratio of not less than 2.
The lyotropic liquid crystal composition can be prepared by a process which comprises the steps of: precipitating silver halide grains from a silver halide emulsion by centrifugation; dispersing again the silver halide grains in water; adding lyotropic liquid crystal into the dispersion; and removing remaining gelatin with an enzyme.
The invention also provides an optically anisotropic thin film comprising lyotropic liquid crystal molecules and silver halide grains or silver grains, said grains having an aspect ratio of not less than 2, wherein the lyotropic liquid crystal molecules and said grains are aligned.
The optically anisotropic thin film can be prepared by a process which comprises coating a lyotropic liquid crystal composition comprising silver halide grains or silver grains dispersed in lyotropic liquid crystal on a support to align the lyotropic liquid crystal and the grains by shearing force applied in a coating procedure, said grains having an aspect ratios of not less than 2.
The invention further provides a lyotropic liquid crystal composition comprising optically anisotropic grains dispersed in lyotropic liquid crystal, said grains having an aspect ratio of not less than 2.
The invention furthermore provides an optically anisotropic thin film comprising lyotropic liquid crystal molecules and optically anisotropic grains, said grains having an aspect ratio of not less than 2, wherein the lyotropic liquid crystal molecules and said grains are aligned.
The optically anisotropic thin film can be prepared by a process which comprises coating a lyotropic liquid crystal composition comprising optically anisotropic grains dispersed in lyotropic liquid crystal on a support to align the lyotropic liquid crystal and the grains by shearing force applied in a coating procedure, said grains having an aspect ratios of not less than 2.
The present invention provides a lyotropic liquid crystal composition stably dispersing fine rod-like materials, and the composition can be easily applied to produce various useful optically anisotropic materials through a coating process. Accordingly, the composition is very useful from the industrial viewpoint.